The current research is designed to follow up on the considerable amount of evidence that 1) indicates that parafollicular (C) cells of the mammalian thyroid gland contains both calcitonin and the biogenic amine, serotonin and 2) supports the hypothesis that thyroidal serotonin may act as an activator of thyroid follicular cells. Studies are in progress dealing with the separation of sheep parafollicular cells from other cells of the thyroid gland. Isolation of parafollicular cell secretory granules for studies aimed at determining the possible role of thyroid serotonin-binding protein in storage organelles. We are also carrying out a study aimed at determining if a direct functional relationship may exist between TSH and thyroidal serotonin and/or serotonin-binding protein. In vitro studies are in progress to determine if the addition of exogenous TSH to the media induces the release of radioactive serotonin-binding protein from thyroid lobes.